1. Field of the Invention
The present invention relates, in general, to an apparatus for measuring the flow rate of a fluid and, more particularly, to a flowmeter suitable for a micro flow sensor.
2. Description of the Related Art
Generally, the accurate measurement of a flow rate in microfluidic systems is very important in various applications, such as chemical component analyzing processes, the administration of medicine, and semiconductor fabrication processes. Micro flow sensors should have very simple structures because they should be installed in a very limited space and be easy to be integrated into microfabrication process of the microfluidic systems. Further, the micro flow sensors should have extremely high measurement accuracy because a flow rate in a micro channel is lower than that in a macro-scale channel. In the last 20 years, various micro flow sensors have been developed using micromachining technology. However, it is difficult to find a sensor simultaneously satisfying the above requirements.
Conventional flow sensors can be classified into mechanical flow sensors (or non-thermal flow sensors) and thermal flow sensors according to the working principles thereof. The mechanical flow sensors measure a flow rate using a pressure drop, a drag force, Coriolis force, etc., which are generated by a flow. The mechanical flow sensors are advantageous in that energy consumption is low and fluid is not heated, while the mechanical flow sensors are problematic in that they are difficult to be integrated into a whole microfluidic system together with a micro valve, pump or the like due to the complex structures thereof. On the contrary, the thermal flow sensors have simple structures and simple electrical characteristics, so that they are preferred to the mechanical flow sensors in microfluidic systems.
The thermal flow sensors measure a flow rate using heat transfer between a heater and a fluid. In this case, conventional thermal flow sensors can be classified into hotwire/film type sensors for measuring the variation of heating power supplied to a sensor or temperature of the sensor, calorimetric type sensors for measuring the asymmetry of temperature profile around the heater, and time-of-flight type sensors for measuring the time required for a heat pulse to travel a certain distance.
In addition to the above-described structural simplicity, the thermal flow sensors have the following advantages. That is, in the case of the hotwire/film type sensors, the response time of the sensor is short, the sensitivity thereof is excellent. Further, in the case of the calorimetric type sensors, since sensors are symmetrically placed in the downstream and the upstream of the heater, a flow direction can be detected.
However, the thermal type flow sensors have the following problems. That is, because the heater must be maintained at a high temperature so as to improve measurement accuracy, fluid around the sensors may be significantly heated. Further, because of the susceptibility of the sensors to the temperature variations of the fluid and the surroundings, the temperature of the fluid must be precisely controlled or the temperature variations must be compensated for. Further, the calorimetric type sensors are limited in a measurement range, and the time-of-flight type sensors are not suitable for the measurement of low speed flow due to heat diffusion.
There have been a lot of efforts to overcome these problems. For example, the heaters were pulsed at a fixed current level in order to avoid thermal drift and the effect of ambient temperature was compensated by inserting resistors into the measurement circuit. To extend the flow measuring range, a thin layer as a heat sink and flow guide was integrated on the backside of sensor or different measurement methods were applied according to the range of flow speed. As the result of the above research and efforts, the disadvantages of the conventional sensors are complemented to some degree, but the structures of the sensors or the measurement circuits become inevitably complicated.
In the meantime, Bedö et al. have proposed a scheme for measuring a flow rate by operating a heater using Alternating Current (AC) power and by measuring a signal using sensors placed on both the upstream and downstream of the heater in a paper entitled “A silicon flow sensor for gases and liquids using AC measurements,” (Sensors and Actuators, Vol. 85, pp. 124–132). This scheme is advantageous in that the response of the sensor to the change of the flow rate becomes faster, and the susceptibility to the surrounding fluid temperature is reduced. However, the above scheme is problematic in that, since the sensor and the heater are separate, heat diffusion and the decay of thermal waves occur while the thermal waves propagate from the heater to the sensor across a flow, so that the sensor must be heated at a very high temperature.